Method of preparing material for lithium secondary battery of high performance

ABSTRACT

Provided is a method for preparing a lithium mixed transition metal oxide, comprising subjecting Li 2 CO 3  and a mixed transition metal precursor to a solid-state reaction under an oxygen-deficient atmosphere with an oxygen concentration of 10 to 50% to thereby prepare a powdered lithium mixed transition metal oxide having a composition represented by Formula I of Li x M y O 2  wherein M, x and y are as defined in the specification. Therefore, since the high-Ni lithium mixed transition metal oxide having a given composition can be prepared by a simple solid-state reaction in air, using a raw material that is cheap and easy to handle, the present invention enables industrial-scale production of the lithium mixed transition metal oxide with significantly decreased production costs and high production efficiency. Further, the thus-produced lithium mixed transition metal oxide is substantially free of impurities, and therefore can exert a high capacity and excellent cycle stability, in conjunction with significantly improved storage stability and high-temperature stability.

FIELD OF THE INVENTION

The present invention relates to a Ni-based lithium mixed transitionmetal oxide and a method for preparing the same. More specifically, thepresent invention relates to a method for preparing a powdered lithiummixed transition metal oxide having a given composition and a specificatomic-level structure which is prepared by a solid-state reaction ofLi₂CO₃ with a mixed transition metal precursor under an oxygen-deficientatmosphere with an oxygen concentration of 10 to 50% by volume.

BACKGROUND OF THE INVENTION

Technological development and increased demand for mobile equipment haveled to a rapid increase in the demand for secondary batteries as anenergy source. Among other things, lithium secondary batteries having ahigh-energy density and voltage, a long cycle lifespan and a lowself-discharge rate are commercially available and widely used.

As cathode active materials for the lithium secondary batteries,lithium-containing cobalt oxide (LiCoO₂) is largely used. In addition,consideration has been made of using lithium-containing manganese oxidessuch as LiMnO₂ having a layered crystal structure and LiMn₂O₄ having aspinel crystal structure, and lithium-containing nickel oxides (LiNiO₂).

Of the aforementioned cathode active materials, LiCoO₂ is currentlywidely used due to superior general properties including excellent cyclecharacteristics, but suffers from low safety, expensiveness due tofinite resources of cobalt as a raw material, and limitations inpractical and mass application thereof as a power source for electricvehicles (EVs) and the like.

Lithium manganese oxides, such as LiMnO₂ and LiMn₂O₄, are abundantresources as raw materials and advantageously employenvironmentally-friendly manganese, and therefore have attracted a greatdeal of attention as a cathode active material capable of substitutingLiCoO₂. However, these lithium manganese oxides suffer from shortcomingssuch as low capacity and poor cycle characteristics.

Whereas, lithium/nickel-based oxides including LiNiO₂ are inexpensive ascompared to the aforementioned cobalt-based oxides and exhibit a highdischarge capacity upon charging to 4.3 V. The reversible capacity ofdoped LiNiO₂ approximates about 200 mAh/g which exceeds the capacity ofLiCoO₂ (about 165 mAh/g). Therefore, despite a slightly lower averagedischarge voltage and a slightly lower volumetric density, commercialbatteries comprising LiNiO₂ as the cathode active material exhibit animproved energy density. To this end, a great deal of intensive researchis being actively undertaken on the feasibility of applications of suchnickel-based cathode active materials for the development ofhigh-capacity batteries. However, the LiNiO₂-based cathode activematerials suffer from some limitations in practical application thereof,due to the following problems.

First, LiNiO₂-based oxides undergo sharp phase transition of the crystalstructure with volumetric changes accompanied by repeatedcharge/discharge cycling, and thereby may suffer from cracking ofparticles or formation of voids in grain boundaries. Consequently,intercalation/deintercalation of lithium ions may be hindered toincrease the polarization resistance, thereby resulting in deteriorationof the charge/discharge performance. In order to prevent such problems,conventional prior arts attempted to prepare a LiNiO₂-based oxide byadding an excess of a Li source and reacting reaction components underan oxygen atmosphere. However, the thus-prepared cathode activematerial, under the charged state, undergoes structural swelling anddestabilization due to the repulsive force between oxygen atoms, andsuffers from problems of severe deterioration in cycle characteristicsdue to repeated charge/discharge cycles.

Second, LiNiO₂ has shortcomings associated with the evolution of anexcess of gas during storage or cycling. That is, in order to smoothlyform the crystal structure, an excess of a Li source is added duringmanufacturing of the LiNiO₂-based oxide, followed by heat treatment. Asa result, water-soluble bases including Li₂CO₃ and LiOH as reactionresidues remain between primary particles and thereby they decompose orreact with electrolytes to thereby produce CO₂ gas, upon charging.Further, LiNiO₂ particles have an agglomerate secondary particlestructure in which primary particles are agglomerated to form secondaryparticles and consequently a contact area with the electrolyte furtherincreases to result in severe evolution of CO₂ gas, which in turnunfortunately leads to the occurrence of battery swelling anddeterioration of the high-temperature safety.

Third, LiNiO₂ suffers from a sharp decrease in the chemical resistanceof a surface thereof upon exposure to air and moisture, and the gelationof slurries by polymerization of a n N-methylpyrrolidone/poly(vinylidenefluoride) (NMP-PVDF) slurry due to a high pH value. These properties ofLiNiO₂ cause severe processing problems during battery production.

Fourth, high-quality LiNiO₂ cannot be produced by a simple solid-statereaction as is used in the production of LiCoO₂, and LiNiMO₂ cathodeactive materials containing an essential dopant cobalt and furtherdopants manganese and aluminum are produced by reacting a lithium sourcesuch as LiOH.H₂O with a mixed transition metal hydroxide under an oxygenor syngas atmosphere (i.e, a. CO₂-deficient atmosphere), whichconsequently increases production costs. Further, when an additionalstep, such as intermediary washing or coating, is included to removeimpurities in the production of LiNiO₂, this leads to a further increasein production costs.

Many prior arts focus on improving properties of LiNiO₂-based cathodeactive materials and processes to prepare LiNiO₂. However, variousproblems, such as high production costs, swelling due to gas evolutionin the fabricated batteries, poor chemical stability, high pH and thelike, have not been sufficiently solved. A few examples will beillustrated hereinafter.

U.S. Pat. No. 6,040,090 (T. Sunagawa et al., Sanyo) discloses a widerange of compositions including nickel-based and high-Ni LiMO₂, thematerials having high crystallinity and being used in Li-ion batteriesin ethylene carbonate (EC) containing an electrolyte. Samples wereprepared on a small scale, using LiOH.H₂O as a lithium source. Thesamples were prepared in a flow of synthetic air composed of a mixtureof oxygen and nitrogen, free of CO₂.

U.S. Pat. No. 5,264,201 (J. R. Dahn et al.) discloses a doped LiNiO₂substantially free of lithium hydroxides and lithium carbonates. Forthis purpose, lithium hydroxide and LiOH.H₂O as a lithium source areemployed and heat treatment is performed under an oxygen atmosphere freeof CO₂, additionally with a low content of H₂O. An excess of lithium“evaporates”; however, “evaporation” is a lab-scale effect and not anoption for large-scale preparation. That is, when applied to alarge-scale production process, it becomes difficult to evaporate anexcess of lithium, thereby resulting in problems associated with theformation of lithium hydroxides and lithium carbonates.

U.S. Pat. No. 5,370,948 (M. Hasegawa et al., Matsushita) discloses aprocess for the production of Mn-doped LiNi_(1−x)Mn_(x)O₂ (x<0.45),wherein the manganese source is manganese nitrate, and the lithiumsource is either lithium hydroxide or lithium nitrate.

U.S. Pat. No. 5,393,622 (Y. Nitta et al., Matsushita) discloses aprocess to prepare LiNi_(1−x)Mn_(x)O₂ by a two-step heating, involvingpre-drying, cooking and the final heating. The final heating is done inan oxidizing gas such as air or oxygen. This patent focuses on oxygen.The disclosed method uses a very low temperature of 550 to 650° C. forcooking, and less than 800° C. for sintering. At higher temperatures,samples deteriorate dramatically. Excess lithium is used such that thefinal samples contain a large amount of soluble bases (i.e., lithiumcompounds). According to the research performed by the inventors of thepresent invention, the observed deterioration is attributable to thepresence of lithium salts as impurities and melting at about 700 to 800°C., thereby detaching the crystallites.

WO 9940029 A1 (M. Benz et al., H. C. Stack) describes a complicatedpreparation method very different from that disclosed in the presentinvention. This preparation method involves the use of lithium nitratesand lithium hydroxides and recovering the evolved noxious gasses. Thesintering temperature never exceeds 800° C. and typically is far lower.

U.S. Pat. No. 4,980,080 (Lecerf, SAFT) describes a process to prepareLiNiO₂-based cathodes from lithium hydroxides and metal oxides attemperatures below 800° C.

In prior arts including the above, LiNiO₂-based cathode active materialsare generally prepared by high cost processes, in a specific reactionatmosphere, especially in a flow of synthetic gas such as oxygen orsynthetic air, free of CO₂, and using LiOH.H₂O, Li-nitrate, Li acetate,etc., but not the inexpensive, easily manageable Li₂CO₃. Furthermore,the final cathode active materials have a high content of soluble bases,originating from carbonate impurities present in the precursors, whichremain in the final cathode because of the thermodynamic limitation.Further, the crystal structure of the final cathode active materials perse is basically unstable even when the final cathode active materialsare substantially free of soluble bases. Consequently, upon exposure toair containing moisture or carbon dioxide during storage of the activematerials, lithium is released to surfaces from the crystal structureand reacts with air to thereby result in continuous formation of solublebases.

Meanwhile, Japanese Unexamined Patent Publication Nos. 2004-281253,2005-150057 and 2005-310744 disclose oxides having a composition formulaof Li_(a)Mn_(x)Ni_(y)M_(z)O₂ (M=Co or Al, 1≦a≦1.2, 0≦x≦0.65, 0.35≦y≦1,0≦z≦0.65, and x+y+z=1). These inventions provide a method of preparingthe oxide involving mixing each transition metal precursor with alithium compound, grinding, drying and sintering the mixture, andre-grinding the sintered composite oxide by ball milling, followed byheat treatment. In addition, working examples disclosed in the aboveprior art employ substantially only LiOH as a lithium source. Further,it was found through various experiments conducted by the inventors ofthe present invention that the aforesaid prior art composite oxidesuffers from significant problems associated with a high-temperaturesafety, due to production of large amounts of impurities such as Li₂CO₃.

Alternatively, encapsulation of high Ni—LiNiO₂ by SiO_(x) protectivecoating has been proposed (H. Omanda, T. Brousse, C. Marhic, and D. M.Schleich, J. Electrochem. Soc. 2004, 151, A922.), but the resultingelectrochemical properties are very poor. In this connection, theinventors of the present invention have investigated the encapsulationby LiPO₃ glass. Even where a complete coverage of the particle isaccomplished, a significant improvement of air-stability could not bemade and electrochemical properties were poor.

Therefore, there is a strong need for the development of a method ofpreparing LiNiO₂-based cathode active materials that can be produced ata low cost from inexpensive precursors, have low contents of solublebases, and show improved properties such as low swelling when applied tocommercial lithium secondary batteries, improved chemical and structuralstability, superior cycle characteristics, and high capacity.

SUMMARY OF THE INVENTION

Therefore, the present invention has been made in view of the aboveproblems and other technical problems that have yet to be resolved.

As a result of a variety of extensive and intensive studies andexperiments in view of the problems as described above, the inventors ofthe present invention provide herewith a lithium mixed transition metaloxide having a given composition, prepared by a solid-state reactionunder an oxygen-deficient atmosphere and using a precursor material thatis cheap and easy to handle, with which it is possible to realizeenvironmental friendliness of the preparation method, decreasedproduction costs and improved production efficiency, where thethus-prepared lithium mixed transition metal oxide is substantially freeof impurities and has superior thermal stability due to a stableatomic-level structure, and a secondary battery comprising such alithium mixed transition metal oxide having a high capacity, excellentcycle characteristics, significantly improved storage properties andhigh-temperature safety. The present invention has been completed basedon these findings.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic view showing a crystal structure of a conventionalNi-based lithium transition metal oxide;

FIG. 2 is a schematic view showing a crystal structure of a Ni-basedlithium mixed transition metal oxide prepared by a method according toone embodiment;

FIGS. 3 and 4 are graphs showing a preferred composition range of aNi-based lithium mixed transition metal oxide prepared by a methodaccording to an embodiment;

FIG. 5 is an FESEM (Field Emission Scanning Electron Microscope) image(×2000) showing LiNiMO₂ according to Example 1. 5A: 850° C.; 5B: 900°C.; 5C: 950° C.; and 5D: 1,000° C.;

FIG. 6 is an FESEM image showing commercial LiMO₂ (M=Ni_(0.8)CO_(0.2))according to Comparative Example 1. 6A: FESEM image of a sample asreceived, and 6B: FESEM image of a sample after heating to 850° C. inair;

FIG. 7 is an FESEM image showing the standard pH titration curve ofcommercial high-Ni LiNiO₂ according to Comparative Example 2. A: Sampleas received, B: After heating of a sample to 800° C. under an oxygenatmosphere, and C: Copy of A;

FIG. 8 is a graph showing a pH titration curve of a sample according toComparative Example 3 during storage of the sample in a wet chamber. A:Sample as received, B: After storage of a sample for 17 hrs, and C:After storage of a sample for 3 days;

FIG. 9 is a graph showing a pH titration curve of a sample according toExample 2 during storage of the sample in a wet chamber. A: Sample asreceived, B: After storage of a sample for 17 hrs, and C: After storageof a sample for 3 days;

FIG. 10 is a graph showing lengths of a-axis and c-axis ofcrystallographic unit cells of samples having different ratios of Li:Min Experimental Example 3;

FIG. 11 is an SEM image of a sample according to Example 4;

FIG. 12 shows the Rietveld refinement on X-ray diffraction patterns of asample according to Example 4;

FIG. 13 is an SEM micrograph (×5000) of a precursor in Example 5, whichis prepared by an inexpensive ammonia-free process and has a lowdensity;

FIG. 14 is a graph showing electrochemical properties of LiNiMO₂according to the present invention in Experimental Example 1. 12A: Graphshowing voltage profiles and rate characteristics at room temperature (1to 7 cycles); 7B: Graph showing cycle stability at 25° C. and 60° C. anda rate of C/5 (3.0 to 4.3V); and 7C: Graph showing discharge profiles(at C/10 rate) for Cycle 2 and Cycle 31, obtained during cycling at 25°C. and 60° C.;

FIG. 15 is a graph showing DSC (differential scanning calorimetry)values for samples of Comparative Examples 3 and 4 in ExperimentalExample 2. A: Commercial Al/Ba-modified LiNiO₂ of Comparative Example 3,and B: Commercial AlPO₄-coated LiNiO₂ of Comparative Example 4;

FIG. 16 is a graph showing DSC values for LiNiMO₂ according to Example 3in Experimental Example 2;

FIG. 17 is a graph showing electrophysical properties of a polymer cellaccording to one embodiment in Experimental Example 3; and

FIG. 18 is a graph showing swelling of a polymer cell duringhigh-temperature storage in Experimental Example 3.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with an aspect of the present invention, the above can beaccomplished by the provision of a method for preparing a lithium mixedtransition metal oxide, comprising subjecting Li₂CO₃ and a mixedtransition metal precursor to a solid-state reaction under anoxygen-deficient atmosphere with an oxygen concentration of 10 to 50% byvolume to thereby prepare a lithium mixed transition metal oxide havinga composition represented by Formula I below:Li_(x)M_(y)O₂  (I)

wherein:

M=M′_(1−k)A_(k), wherein M′ is Ni_(1−a−b)(Ni_(1/2)Mn_(1/2))_(a)Co_(b),0.65≦a+b≦0.85 and 0.1≦b≦0.4;

A is a dopant;

0≦k≦0.05; and

x+y≈2 and 0.95≦x≦1.05.

Therefore, since a high-Ni lithium mixed transition metal oxide having agiven composition is prepared by a simple solid-state reaction under anoxygen deficient atmosphere including for example, air, using a rawmaterial that is cheap and easy to handle, so the present invention canenables industrial-scale production of the lithium mixed transitionmetal oxide with significantly decreased production costs and highproduction efficiency. Further, the high-Ni lithium mixed transitionmetal oxide produced according to the method has a very stableatomic-level structure and is substantially free of water-soluble basessuch as Li₂CO₃. Therefore, the lithium mixed transition metal oxide ofthe present invention can exert excellent storage stability, decreasedgas evolution and thereby excellent high-temperature stability, and asecondary battery comprising such a lithium mixed transition metal oxidecan exert a high capacity and high cycle stability.

As used herein, the term “high-Ni” means that a content of nickel isrelatively high, relative to other transition metals present whichconstitute the lithium mixed transition metal oxide, such as nickel,manganese, cobalt, and the like. Hereinafter, where appropriatethroughout the specification, the term “high-Ni lithium mixed transitionmetal oxide in accordance with the present invention” is usedinterchangeably with the term “LiNiMO₂”. Therefore, NiM in LiNiMO₂ is asuggestive expression representing a complex composition of Ni, Mn andCo in Formula I.

Conventional Ni-based cathode active materials contain large amounts ofwater-soluble bases such as lithium oxides, lithium sulfates, lithiumcarbonates (Li₂CO₃), and the like. These water-soluble bases may bebases, such as Li₂CO₃ and LiOH, present in LiNiMO₂, or otherwise may bebases produced by ion exchange (H⁺ (water)← →Li⁺ (surface, an outersurface of the bulk)), performed at the surface of LiNiMO₂. The bases ofthe latter case are usually present at a negligible level.

The former water-soluble bases may be produced due to the presence ofunreacted lithium raw materials upon sintering of lithium mixedtransition metal oxides. This is because as production of conventionallithium mixed transition metal oxides involves an addition of relativelylarge amounts of lithium and a low-temperature sintering process so asto prevent the disintegration of a layered crystal structure, theresulting particles have relatively large amounts of grain boundaries ascompared to the cobalt-based oxides, and a sufficient reaction oflithium ions is not realized upon sintering.

In addition, even when an initial amount of Li₂CO₃ is low, Li₂CO₃ mayalso be produced during fabrication of the battery or storage ofelectrode active materials. These water-soluble bases react withelectrolytes in the battery to thereby cause gas evolution and batteryswelling, which consequently result in severe deterioration of thehigh-temperature safety.

On the other hand, since the cathode active material prepared by themethod in accordance with the present invention stably maintains thelayered crystal structure by a specific composition of transition metalelements and a reaction atmosphere, despite the use of Li₂CO₃ as a rawmaterial, it is possible to carry out the sintering process at ahigh-temperature, thereby resulting in small amounts of grainboundaries. In addition, as retention of unreacted lithium on surfacesof particles is prevented, the particle surfaces are substantially freeof water-soluble bases such as lithium carbonates, lithium sulfates, andthe like. In the present invention, a content of water-soluble basessuch as Li₂CO₃ includes all of Li₂CO₃ remaining upon production of thelithium mixed transition metal oxide, or Li₂CO₃ produced duringfabrication of the battery or storage of electrode active materials.

In the present invention, the content of the water-soluble bases ismeasured by pH titration. As used herein, the phrase “is (are)substantially free of water-soluble bases” refers to an extent that upontitration of 200 mL of a solution containing the lithium mixedtransition metal oxide with 0.1M HCl, a HCl solution used to reach a pHof less than 5 is preferably consumed in an amount of less than 20 mL,more preferably less than 10 mL. Herein, 200 mL of the aforementionedsolution contains substantially all kinds of the water-soluble bases inthe lithium mixed transition metal oxide, and is prepared by repeatedlysoaking and decanting 10 g of the lithium mixed transition metal oxide.At this time, there are no significant influences of parameters such asa total soaking time of the powder in water. In addition, the content ofthe water-soluble bases may be preferably below 0.07% by weight of thecathode active material.

One of the important features of the present invention is that a desiredlithium mixed transition metal oxide is prepared by a solid-statereaction of Li₂CO₃ and a mixed transition metal precursor under anoxygen-deficient atmosphere.

In this connection, it was found through various experiments conductedby the inventors of the present invention that when conventionalhigh-nickel LiMO₂ is sintered in air containing a trace amount of CO₂,LiMO₂ decomposes with a decrease of Ni³⁺ as shown in the followingreaction below, during which amounts of Li₂CO₃ impurities increase.LiM³⁺O₂+CO₂ +aLi_(1+x)M_(1+x1) ^(3+,2+)O₂ +bLi₂CO₃ +cO₂

This is believed to be due to that the decomposition of some Ni³⁺ intoNi²⁺ upon sintering results in destabilization of the crystal structure,which consequently leads to an oxide form having excessive cationmixing, i.e. Li-deficient form of Li_(1−a)Ni_(1+a)O₂ having transitionmetal cations misplaced on lithium sites of the crystal structure, andlithium ions, released from partial collapse of the crystal structure,react with CO₂ in air.

For these reasons, the conventional prior art suffered from problems inthat the use of Li₂CO₃ as a raw material brings about the evolution ofCO₂ due to decomposition of Li₂CO₃, which then thermodynamically hindersfurther decomposition of Li₂CO₃ necessary for the reaction even at a lowpartial pressure of CO₂, consequently resulting in no furtherprogression of the reaction. In addition, an excessive addition ofLi₂CO₃ is accompanied by a problem of residual Li₂CO₃ after thereaction.

Therefore, in order to prevent such problems associated with thelithium-deficiency and cation mixing and in order to increase a relativeamount of Ni³⁺, conventional prior arts conducted the productionreaction using an excessive amount of LiOH.H₂O as a lithium source, witha ratio of M(OH)₂ and Li of 1:1.05 to 1.15 (M(OH)₂:Li-compound) under ahigh-oxygen atmosphere.

However, LiOH.H₂O (technical grade) contains primarily >1% Li₂CO₃ byweight of impurities that are not decomposed or removed during thesintering process under an oxygen atmosphere and therefore remain in thefinal product. Further, an excess of the residual Li₂CO₃ accelerates theelectrolyte decomposition to thereby result in the evolution of gas.Therefore, the conventional method suffered from various problems suchas disintegration of secondary particles into single primarycrystallites, lowered storage stability, and deterioration of thehigh-temperature safety resulting from the gas evolution due to thereaction of the residual Li₂CO₃ with the electrolyte in the battery.

Further, the lithium mixed transition metal oxide prepared by aconventional method has a layered crystal structure as shown in FIG. 1,and desertion of lithium ions from the reversible lithium layers in thecharged state brings about swelling and destabilization of the crystalstructure due to the repulsive force between oxygen atoms in the MOlayers (mixed-transition metal oxide layers), thus suffering from theproblems associated with sharp decreases in the capacity and cyclecharacteristics, resulting from changes in the crystal structure due torepeated charge/discharge cycles.

As a result of a variety of extensive and intensive studies andexperiments, the inventors of the present invention have found that whenthe lithium mixed transition metal oxide is prepared by a solid-statereaction of Li₂CO₃ with the mixed transition metal precursor under anoxygen-deficient atmosphere, it is possible to produce a cathode activematerial containing the lithium mixed transition metal oxidesubstantially free of Li₂CO₃.

Specifically, under the oxygen-deficient atmosphere, desorption of someoxygen atoms takes place from the MO layers, which leads to a decreasein an oxidation number of Ni, thereby increasing amounts of Ni²⁺ ions.As a result, some of the Ni²⁺ ions are inserted into the reversiblelithium layers, as shown in FIG. 2. However, contrary to conventionallyknown or accepted ideas in the related art thatintercalation/deintercalation of lithium ions will be hindered due tosuch insertion of Ni²⁺ ions into the reversible lithium layers, aninsertion of an effective amount of Ni²⁺ ions can preventdestabilization of the crystal structure that may occur due to therepulsive force between oxygen atoms in the MO layers, upon charge. Asused herein, “an effective amount of Ni²⁺ ions”, can include about 3 toabout 7 mole percent of the total amount of Ni ions present. Therefore,stabilization of the crystal structure is achieved to result in nooccurrence of further structural collapse by oxygen desorption. Further,it is believed that the lifespan characteristics and safety aresimultaneously improved, due to no further formation of Ni²⁺ ions withmaintenance of the oxidation number of Ni ions inserted into thereversible lithium layers, even when lithium ions are released during acharge process. Hence, it can be said that such a concept of the presentinvention is a remarkable one which is completely opposite to andoverthrows the conventional idea.

Thus, the present invention can fundamentally prevent the problems thatmay occur due to the presence of the residual Li₂CO₃ in the finalproduct (active material), and provides a highly economical process byperforming the production reaction using a relatively small amount ofinexpensive Li₂CO₃ as a reactant and an oxygen-deficient atmosphere suchas air. Further, the sintering and storage stabilities are excellent dueto the stability of the crystal structure, and thereby the batterycapacity and cycle characteristics can be significantly improvedsimultaneously with a desired level of rate characteristics.

However, under an atmosphere with excessive oxygen-deficiency, anexcessive amount of Ni²⁺ ions transfer to the reversible lithium layersduring a synthesis process, thereby resulting in hindrance of theintercalation/deintercalation of lithium ions, and therefore theperformance of the battery cannot be exerted sufficiently. On the otherhand, if the oxygen concentration is excessively high, a desired amountof Ni²⁺ ions cannot be inserted into the reversible lithium layers.Taking into consideration such problems, the synthetic reaction may becarried out under an atmosphere with an oxygen concentration ofpreferably 10% to 50% by volume, and more preferably 10% to 30% byvolume. In a specific embodiment, the reaction may be carried out underan air atmosphere.

Another feature of the present invention is that raw materials producedby an inexpensive or economical process and being easy to handle can beused, and particularly Li₂CO₃ which is difficult to employ in the priorart can be used itself as a lithium source.

As an added amount of Li₂CO₃ as the lithium source decreases, that is, aratio (Li/M) of lithium to the mixed transition metal source (M)decreases, an amount of Ni inserted into the MO layers graduallyincreases. Therefore, if excessive amounts of Ni ions are inserted intothe reversible lithium layers, a movement of Li ions duringcharge/discharge processes is hampered, which thereby leads to problemsassociated with a decrease in the capacity or deterioration of the ratecharacteristics. On the other hand, if an added amount of Li₂CO₃ isexcessively large, that is, the Li/M ratio is excessively high, theamount of Ni inserted into the reversible lithium layers is excessivelylow, which can undesirably lead to structural instability, therebypresenting decreased safety of the battery and poor lifespancharacteristics. Further, at a high Li/M value, amounts of unreactedLi₂CO₃ increase to thereby result in a high pH-titration value, i.e.production of large amounts of impurities, and consequently thehigh-temperature safety can deteriorate.

Therefore, in one embodiment, an added amount of Li₂CO₃ as the lithiumsource may be from 0.95 to 1.04:1 where the ratio of Li₂CO₃:mixedtransition metal raw material, is a w/w ratio, based on the weight ofthe mixed transition metal as the other raw material.

As a result, the product is substantially free of impurities due to alack of surplus Li₂CO₃ in the product (the cathode active material) byadding a stoichiometric amount (i.e., by not adding an excess) of thelithium source, so there are no problems associated with residual Li₂CO₃and a relatively small amount of inexpensive Li₂CO₃ is used to therebyprovide a highly economical process.

As the mixed transition metal precursor, M(OH)₂ or MOOH (M is as definedin Formula I) can specifically be used. As used herein, the term “mixed”means that several transition metal elements are well mixed at theatomic level.

In prior art processes, as the mixed transition metal precursors,mixtures of Ni-based transition metal hydroxides are generally employed.However, these materials commonly contain carbonate impurities. This isbecause Ni(OH)₂ is prepared by co-precipitation of a Ni-based salt suchas NiSO₄ with a base such as NaOH in which the technical grade NaOHcontains Na₂CO₃ and the CO₃ anion is more easily inserted into theNi(OH)₂ structure than the OH anion.

Further, in order to increase an energy density of the cathode activematerial, conventional prior art processes employed MOOH having a hightap density of 1.5 to 3.0. However, the use of such a high-tap densityprecursor makes it difficult to achieve the incorporation of thereactant (lithium) into the inside of the precursor particles during thesynthetic process, which then lowers the reactivity to thereby result inproduction of large amounts of impurities. Further, for preparation ofMOOH having a high tap density, co-precipitation of MSO₄ and NaOH shouldbe carried out in the presence of excess ammonia as a complexingadditive. However, ammonia in waste water causes environmental problemsand thus is strictly regulated. It is, however, not possible to preparethe mixed oxyhydroxide having a high density by an ammonia-free processthat is less expensive, is more environmentally friendly and is easierto proceed this process.

However, according to the research performed by the inventors of thepresent invention, it was found that even though the mixed transitionmetal precursor prepared by the ammonia-free process exhibits arelatively low tap density, a lithium mixed transition metal oxideprepared using the thus-prepared precursor which has an excellentsintering stability makes it is possible to prepare a mixed transitionmetal oxide having a superior reactivity.

In this way, the lithium mixed transition metal oxide, which wasprepared by the method according to the present invention, as discussedhereinbefore, can maintain a well-layered crystal structure due to theinsertion of some Ni ions into the reversible lithium layers, thusexhibiting excellent sintering stability. Accordingly, the presentinvention can employ the mixed transition metal precursor having a lowtap density, as the raw material.

Therefore, since the raw material, i.e. the mixed transition metalprecursor, is environmentally friendly, can be easily prepared at lowproduction costs and also has a large volume of voids between primaryparticles, e.g. a low tap density, it is possible to easily realize theintroduction of the lithium source into the inside of the precursorparticles, thereby improving the reactivity, and it is also possible toprevent production of impurities and reduce an amount of the lithiumsource (Li₂CO₃) to be used, so the method of the present invention ishighly economical.

As used herein, the term “ammonia-free process” means that only NaOHwithout the use of aqueous ammonia is used as a co-precipitating agentin a co-precipitation process of a metal hydroxide. That is, thetransition metal precursor is obtained by dissolving a metal salt MSO₄(M is a metal of a composition to be used) in water, and graduallyadding a small amount of a precipitating agent NaOH with stirring. Atthis time, the introduction of ammonia lowers the repulsive forcebetween particles to thereby result in densification of co-precipitatedparticles, which then increases a density of particles. However, when itis desired to obtain a hydroxide having a low tap density as in thepresent invention, there is no need to employ ammonia.

In one specific embodiment, the tap density of the mixed transitionmetal precursor can be from 1.1 to 1.6 g/cm³. If the tap density isexcessively low, a chargeable amount of the active material decreases,so the capacity per volume may be lowered. On the other hand, if the tapdensity is excessively high, the reactivity with the lithium sourcematerial is lowered and therefore impurities may be undesirably formed.

The solid-state reaction includes a sintering process preferably at 600to 1,100° C. for 3 to 20 hours, and more preferably 800 to 1,050° C. for5 to 15 hours. If the sintering temperature is excessively high, thismay lead to non-uniform growth of particles, and reduction of the volumecapacity of the battery due to a decreased amount of particles that canbe contained per unit area, arising from an excessively large size ofparticles. On the other hand, if the sintering temperature isexcessively low, an insufficient reaction leads to the retention of theraw materials in the particles, thereby presenting the risk of damagingthe high-temperature safety of the battery, and it may be difficult tomaintain a stable structure, due to the deterioration of the volumedensity and crystallinity. Further, if the sintering time is too short,it is difficult to obtain a lithium nickel-based oxide having highcrystallinity. On the other hand, if the sintering time is too long,this may undesirably lead to excessively large particle diameter andreduced production efficiency.

The method in accordance with the present invention enables theproduction of a desired lithium transition metal oxide by a single heattreatment and is thus also desirable in terms of economic efficiency ofthe manufacturing process.

In addition, various parameters may occur as the process for preparationof the lithium mixed transition metal oxide is scaled-up. A few grams ofsamples in a furnace behave very differently from a few kg of samples,because the gas transport kinetics at a low partial pressure iscompletely different. Specifically, in a small-scale process, Lievaporation occurs and CO₂ transport is fast, whereas in a large-scaleprocess, these processes are retarded. Where the Li evaporation and CO₂transport are retarded, a gas partial pressure in the furnace increases,which in turn hinders further decomposition of Li₂CO₃ necessary for thereaction, consequently resulting in retention of the unreacted Li₂CO₃,and the resulting LiNiMO₂ decomposes to result in the destabilization ofthe crystal structure.

Accordingly, when it is desired to prepare the lithium mixed transitionmetal oxide on a large-scale using the method of the present invention,the preparation process is specifically carried out under a high rate ofair circulation. As used herein, the term “large scale” means that asample has a size of 5 kg or more because similar behavior is expectedin 100 kg of sample when the process has been correctly scaled-up, i.e.,a similar gas flow (m³/kg of sample) reaches 100 kg of sample.

In order to achieve high air circulation upon the production of thelithium transition metal oxide by the large-scale mass productionprocess, preferably at least 2 m³ (volume at room temperature) of air,and more preferably at least 10 m³ of air, per kg of the final lithiummixed transition metal oxide, may be pumped into or out of a reactionvessel. As such, even when the present invention is applied to alarge-scale production process, it is possible to prepare the lithiummixed transition metal oxide which is substantially free of impurities.

In an embodiment of the present invention, a heat exchanger may beemployed to minimize energy expenditure upon air circulation bypre-warming the in-flowing air before it enters the reaction vessel,while cooling the out-flowing air.

In a specific example, air flow of 2 m³/kg corresponds to about 1.5 kgof air at 25° C. The heat capacity of air is about 1 kJ/kg° K and thetemperature difference is about 800K. Thus, at least about 0.33 kWh isrequired per kg of the final sample for air heating. Where the air flowis 10 m³, about 2 kWh is then necessary. Thus, the typical additionalenergy cost amounts to about 2 to 10% of the total cathode sales price.The additional energy cost can be significantly reduced when theair-exchange is made by using a heat exchanger. In addition, the use ofthe heat exchanger can also reduce the temperature gradient in thereaction vessel. To further decrease the temperature gradient, it isrecommended to provide several air flows into the reaction vesselsimultaneously.

In accordance with another aspect of the present invention, there isprovided a lithium mixed transition metal oxide prepared by theaforementioned method, and a cathode active material for a secondarybattery comprising the same.

The lithium mixed transition metal oxide in accordance with the presentinvention can maintain a well-layered structure due to the insertion ofMO layer (mixed-transition metal oxide layers)-derived Ni²⁺ ions intoreversible lithium layers (lithium intercalation/deintercalationlayers), even when lithium ions are released during a charge process. Asa result, the lithium mixed transition metal oxide exhibits veryexcellent sintering stability and no occurrence of Li₂CO₃ impuritiesresulting from reduction and decomposition of Ni³⁺ ions, and issubstantially free of water-soluble bases such as lithium carbonates andlithium sulfates. Accordingly, the lithium mixed transition metal oxideof the present invention exhibits excellent storage stability, decreasedgas evolution and thereby excellent high-temperature stabilitysimultaneously with the feasibility of industrial-scale production atlow production costs.

The cathode active material in accordance with the present invention maybe comprised only of the lithium mixed transition metal oxide having theabove-specified composition and the specific atomic-level structure or,where appropriate, it may be comprised of the lithium mixed transitionmetal oxide in conjunction with other lithium-containing transitionmetal oxides.

Examples of the lithium-containing transition metal oxides that can beused in the present invention can include, but are not limited to,layered compounds such as lithium cobalt oxide (LiCoO₂) and lithiumnickel oxide (LiNiO₂), or compounds substituted with one or moretransition metals; lithium manganese oxides such as compounds of FormulaLi_(1+y)Mn_(2−y)O₄ (0≦y≦0.33), LiMnO₃, LiMn₂O₃ and LiMnO₂; lithiumcopper oxide (Li₂CuO₂); vanadium oxides such as LiV₃O₈, V₂O₅, andCu₂V₂O₇; Ni-site type lithium nickel oxides of Formula LiNi_(1−y)M_(y)O₂(M=Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and 0.01≦y≦0.3); lithium manganesecomposite oxides of Formula LiMn_(2−y)M_(y)O₂ (M=Co, Ni, Fe, Cr, Zn, orTa, and 0.01≦y≦0.1), or Formula Li₂Mn₃MO₈ (M=Fe, Co, Ni, Cu, or Zn);LiMn₂O₄ wherein a portion of Li is substituted with alkaline earth metalions; disulfide compounds; and Fe₂(MoO₄)₃, LiFe₃O₄, and the like.

In accordance with yet another aspect of the present invention, there isprovided a lithium secondary battery comprising the aforementionedlithium mixed transition metal oxide as a cathode active material. Thelithium secondary battery can comprise a cathode, an anode, a separatorand a lithium salt-containing non-aqueous electrolyte. General methodsfor preparing the lithium secondary battery are known in the art andtherefore detailed description thereof will be omitted herefrom.

EXAMPLES

Now, the present invention will be described in more detail withreference to the following Examples. These examples are provided onlyfor illustrating the present invention and should not be construed aslimiting the scope and spirit of the present invention.

For reference, the content of water-soluble bases contained in thepowder in the working examples was measured according to the followingmethod.

Contents and Characterization of Water-Soluble Bases (pH Titration)

First, 5 g of a cathode active material powder was added to 25 mL ofwater, followed by brief stirring. About 20 mL of a clear solution wasseparated and pooled from the powder by soaking and decanting. Again,about 20 mL of water was added to the powder and the resulting mixturewas stirred, followed by decanting and pooling. The soaking anddecanting were repeated at least 5 times. In this manner, a total of 100mL of the clear solution containing water-soluble bases was pooled. A0.1M HCl solution was added to the thus-pooled solution, followed by pHtitration with stirring. The pH was recorded as a function of time.Experiments were terminated when the pH reached a value of less thanabout 3, and a flow rate was appropriately selected within a range inwhich titration takes about 20 to about 30 min. The content of thewater-soluble bases was measured as an amount of acid that was useduntil the pH reaches a value of less than about 5. Characterization ofwater-soluble bases was made from the pH profile.

Example 1

A mixed oxyhydroxide of Formula MOOH(M=Ni_(4/15)(Mn_(1/2)Ni_(1/2))_(8/15)Co_(0.2)) as a mixed transitionmetal precursor and Li₂CO₃ were mixed in a stoichiometric ratio(Li:M=1.02:1), and the mixture was sintered in air at varioustemperatures of 850 (Ex. 1A), 900 (Ex. 1B), 950 (Ex. 1C), and 1,000° C.(Ex. 1D) for 10 hours, to prepare a lithium mixed transition metaloxide. Herein, secondary particles were maintained intact without beingcollapsed, and the crystal size increased with an increase in thesintering temperature.

X-ray analysis shows that all samples have a well-layered crystalstructure. Further, a unit cell volume did not exhibit a significantchange with an increase in the sintering temperature, thus representingthat there was no significant oxygen-deficiency and no significantincrease of cation mixing, in conjunction with essentially no occurrenceof lithium evaporation.

The crystallographic data for the thus-prepared lithium mixed transitionmetal oxide are given in Table 1 below, and FESEM images thereof areshown in FIG. 5. From these results, it was found that the lithium mixedtransition metal oxide is LiNiMO₂ having a well-layered crystalstructure with the insertion of nickel at a level of 3.9 to 4.5% into areversible lithium layer. Further, it was also found that even thoughLi₂CO₃ was used as a raw material and sintering was carried out in air,proper amounts of Ni²⁺ ions were inserted into the lithium layer,thereby achieving the structural stability.

Particularly, Sample B, sintered at 900° C. (Ex. 1B), exhibited a highc:a ratio and therefore excellent crystallinity, a low unit cell volumeand a reasonable cation mixing ratio. As a result, Sample B showed themost excellent electrochemical properties, and a BET surface area ofabout 0.4 to about 0.8 m²/g. TABLE 1 Example 1(A-D) (A) (B) (C) (D)Sintering 850° C. 900° C. 950° C. 1,000° C. temp. Unit cell 33.902 Å³33.921 Å³ 33.934 Å³ 33.957 Å³ vol. Normalized 1.0123 1.0122 1.01201.0117 c:a ratio c:a/24{circumflex over ( )}0.5 Cation mixing 4.5% 3.9%4.3% 4.5% (Rietveld refinement)

Comparative Example 1

50 g of a commercial sample having a composition ofLiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ represented by Formula LiNi_(1−x)M_(x)O₂(x=0.3, and M=Mn_(1/3)Ni_(1/3)Co_(1/3)) was heated in air to 750° C.(CEx. 1A), 850° C. (CEx. 1B), 900° C. (CEx. 1C) and 950° C. (CEx. 1D)(10 hrs), respectively.

X-ray analysis was carried out to obtain detailed lattice parameterswith high resolution. Cation mixing was observed by Rietveld refinement,and morphology was analyzed by FESEM. The results thus obtained aregiven in Table 2 below. Referring to Table 2, it can be seen that all ofthe samples heated to a temperature of T≧750° C. (CEx. 1A-D) exhibitedcontinuous degradation of a crystal structure (increased cation mixing,increased lattice constant and decreased c:a ratio). FIG. 6 shows aFESEM image of a commercial sample as received and a FESEM image of thesame sample heated to 850° C. (CEx. 1B) in air; and it can be seen thatthe sample heated to a temperature of T≧850° C. (CEx. 1B-D) exhibitedstructural collapse. This is believed to be due to that Li₂CO₃, formedduring heating in air, melts to thereby segregate particles. TABLE 2Comp. Ex. 1 (A-D) (A) (B) (C) (D) Sintering 750° C. 850° C. 900° C. 950°C. temp. Unit cell 33.902 Å³ 33.920 Å³ 33.934 Å³ 33.957 Å³ vol.Normalized 1.0103 1.0100 1.0090 1.0085 c:a ratio c:a/24{circumflex over( )}0.5 Cation mixing 10% 12% 15% 18% (Rietveld refinement)

Therefore, it can be seen that it is impossible to produce aconventional lithium mixed transition metal oxide in the air containingtrace amounts of carbon dioxide, due to thermodynamic limitations. Inaddition, upon producing the lithium mixed transition metal oxideaccording to a conventional method, the use of Li₂CO₃ as a raw materialis accompanied by evolution of CO₂ due to decomposition of Li₂CO₃,thereby leading to thermodynamic hindrance of further decomposition ofLi₂CO₃ necessary for the reaction, consequently resulting in no furtherprogression of the reaction. For these reasons, it was found that such aconventional method cannot be applied to the practical productionprocess.

Comparative Example 2

The pH titration was carried out at a flow rate of >2 L/min for 400 g ofa commercial sample having a composition of LiNi_(0.8)CO_(0.2)O₂. Theresults thus obtained are given in FIG. 7. In FIG. 7, Curve A (CEx. 2A)represents pH titration for the sample as received, and Curve B (CEx.2B) represents pH titration for the sample heated to 800° C. in a flowof pure oxygen for 24 hours. From the analysis results of pH profiles,it can be seen that the contents of Li₂CO₃ before and after heattreatment were the same therebetween, and there was no reaction ofLi₂CO₃ impurities. That is, it can be seen that the heat treatment underan oxygen atmosphere resulted in no additional production of Li₂CO₃impurities, but Li₂CO₃ impurities present in the particles were notdecomposed. Through slightly increased cation mixing, a slightlydecreased c:a ratio and a slightly decreased unit cell volume from theX-ray analysis results, it was found that the content of Li slightlydecreased in the crystal structure of LiNiO₂ in conjunction with theformation of a small amount of Li₂O. Therefore, it can be seen that itis impossible to prepare a stoichiometric lithium mixed transition metaloxide with no impurities and no lithium-deficiency in a flow of oxygengas or synthetic air.

Comparative Example 3

LiAl_(0.02)Ni_(0.78)CO_(0.2)O₂ containing less than 3% by mole of analuminum compound, as commercially available Al/Ba-modified, high-nickelLiNiO₂, was stored in a wet chamber (90% relative humidity; abbreviated“RH”) at 60° C. in air. The pH titration was carried out for a sampleprior to exposure to moisture, and samples wet-stored for 17 hrs and 3days, respectively. The results thus obtained are given in FIG. 8.Referring to FIG. 8, an amount of water-soluble bases was low beforestorage, but substantial amounts of water-soluble bases, primarilycomprising Li₂CO₃, were continuously formed upon exposure to air.Therefore, even when an initial amount of Li₂CO₃ impurities was low, itwas revealed that the commercially available high-nickel LiNiO₂ is notstable in air and therefore rapidly decomposes at a substantial rate,and substantial amounts of Li₂CO₃ impurities are formed during storage.

Example 2

The pH titration was carried out for a sample of the lithium mixedtransition metal oxide in accordance with Example 2 prior to exposure tomoisture, and samples stored in a wet chamber (90% RH) at 60° C. in airfor 17 hours and 3 days, respectively. The results thus obtained aregiven in FIG. 9.

Upon comparing the lithium mixed transition metal oxide of Example 2(see FIG. 9) with the sample of Comparative Example 3 (see FIG. 8), thesample of Comparative Example 3 (stored for 17 hours) exhibitedconsumption of about 20 mL of HCl, whereas the sample of Example 2(stored for 17 hours) exhibited consumption of 10 mL of HCl, thusshowing an about two-fold decrease in production of the water-solublebases. Further, in 3-day-storage samples, the sample of ComparativeExample 3 exhibited consumption of about 110 mL of HCl, whereas thesample of Example 2 exhibited consumption of 26 mL of HCl, whichcorresponds to an about five-fold decrease in production of thewater-soluble bases. Therefore, it can be seen that the sample ofExample 2 decomposed at a rate about five-fold slower than that of thesample of Comparative Example 3. Then, it can be shown that the lithiummixed transition metal oxide of Example 2 exhibits superior chemicalresistance even when it is exposed to air and moisture.

Comparative Example 4

A high-nickel LiNiO₂ sample having a composition ofLiNi_(0.8)Mn_(0.05)CO_(0.15)O₂, as a commercial sample which wassurface-coated with AlPO₄ followed by gentle heat treatment, wassubjected to pH titration before and after storage in a wet chamber. Asa result of pH titration, 12 mL of 0.1M HCl was consumed per 10 gcathode, and the content of Li₂CO₃ after storage was slightly lower (80to 90%) as compared to the sample of Comparative Example 3, but thecontent of Li₂CO₃ was higher than that of Example 2. Consequently, itwas found that the aforementioned high-Ni LiNiO₂ shows no improvementsin the stability against exposure to the air even when it wassurface-coated, and also exhibits insignificant improvements in theelectrochemical properties such as the cycle stability and ratecharacteristics.

Example 3

Samples with different Li:M ratios were prepared from MOOH(M=Ni_(4/15)(Mn_(1/2)Ni_(1/2))_(8/15)Cu_(0.2)). Li₂CO₃ was used as alithium source. Specifically, 7 samples each of about 50 g with Li:Mratios ranging from 0.925 to 1.12 were prepared by a sintering processin air at a temperature of 910 to 920° C. Then, electrochemicalproperties were tested.

Table 3 below provides the obtained crystallographic data. The unit cellvolume changes smoothly according to the Li:M molar ratio. FIG. 10 showsits crystallographic map. All samples are located on a straight line.According to the results of pH titration, the content of soluble baseslightly increased with an increase of the Li:M ratio, but the totalamount thereof was small. Accordingly, the soluble base probablyoriginates from the surface basicity (i.e., is present by an ionexchange mechanism) but not from the dissolution of Li₂CO₃ impurities asobserved in Comparative Example 1.

Therefore, this experiment clearly shows that the lithium mixedtransition metal oxide prepared by the method in accordance with thepresent invention is in the Li stoichiometric range and additional Li isinserted into the crystal structure. In addition, it can be seen thatstoichiometric samples without Li₂CO₃ impurity can be obtained even whenLi₂CO₃ is used as a precursor and the sintering is carried out in air.

That is, as the Li/M ratio decreases, the amount of Ni²⁺ inserted intothe reversible lithium layer gradually increases. Insertion ofexcessively large amounts of Ni²⁺ into the reversible lithium layerhinders the movement of Li⁺ during the charge/discharge process, therebyresulting in decreased capacity or poor rate characteristics. On theother hand, if the Li/M ratio is excessively high, the amount of Ni²⁺inserted into the reversible lithium layer is too low, which may resultin structural instability leading to deterioration of the battery safetyand lifespan characteristics. Further, at the high Li/M value, amountsof unreacted Li₂CO₃ increase to thereby result in a high pH-titrationvalue. Therefore, upon considering the performance and safety of thebattery, the ratio of Li:M is particularly preferably in a range of 0.95to 1.04 (Samples B, C and D) to ensure that the value of Ni²⁺ insertedinto the lithium layer is in a range of 3 to 7%. TABLE 3 Samples A B C DE F G Li:M ratio 0.925 0.975 1.0 1.025 1.05 1.075 1.125 Unit cell vol.34.111 Å³ 34.023 Å³ 33.923 Å³ 33.921 Å³ 33.882 Å³ 33.857 Å³ 33.764 Å³c:a ratio 1.0116 1.0117 1.0119 1.0122 1.0122 1.0123 1.0125 Cation mixing8.8% 6.6% 4.7% 4.0% 2.1% 2.5% 1.4% pH 3 3.5 5 9 15 19 25

Example 4

Li₂CO₃ and a mixed oxyhydroxide of Formula MOOH(M=Ni_(4/15)(Mn_(1/2)Ni_(1/2))_(8/15)Co_(0.2)) were introduced into afurnace with an about 20 L chamber and sintered at 920° C. for 10 hours,during which more than 10 m³ of air was pumped into the furnace, therebypreparing about 5 kg of LiNiMO₂ in one batch.

After sintering was complete, a unit cell constant was determined byX-ray analysis, and a unit cell volume was compared with a target value(Sample B of Example 1: 33.921 Å³). ICP analysis showed that a ratio ofLi and M is very close to 1.00, and the unit cell volume was within thetarget range. FIG. 11 shows a scanning electron microscope (SEM) imageof the thus-prepared cathode active material and FIG. 12 shows resultsof Rietveld refinement. Referring to these drawings, it can be seen thatthe sample exhibits high crystallinity and well-layered structure, amole fraction of Ni²⁺ inserted into a reversible lithium layer is 3.97%,and the calculated value and the measured value of the mole fraction ofNi²⁺ are approximately the same.

Meanwhile, upon performing pH titration, less than 10 mL of 0.1M HCl wasconsumed to titrate 10 g of a cathode to achieve a pH of less than 5,which corresponds to a Li₂CO₃ impurity content of less than about 0.035wt %. Hence, these results show that it is possible to achieve massproduction of substantially Li₂CO₃-free LiNiMO₂ having a stable crystalstructure from the mixed oxyhydroxide and Li₂CO₃ by a solid-statereaction.

Example 5

More than 1 kg of MOOH (M=Ni_(4/15)(Mn_(1/2)Ni_(1/2))_(8/15)Co_(0.2))was prepared by ammonia-free coprecipitation of MSO₄ and NaOH at 80° C.under the pH-adjustment condition. FIG. 13 shows an SEM micrograph ofthe thus-prepared precursor hydroxide. The aforementioned MOOH exhibiteda narrow particle diameter distribution, and a tap density of about 1.2g/cm³. A lithium mixed transition metal oxide was prepared using MOOH asa precursor. Sintering was carried out at 930° C. The lithium mixedtransition metal oxide prepared using such a precursor did not exhibitthe disintegration of particles as shown in Comparative Example 2.Therefore, from the excellent sintering stability of LiMO₂, it can beseen that LiMO₂ can be prepared from the mixed oxyhydroxide having a lowtap density.

Experimental Example 1 Test of Electrochemical Properties

Coin cells were fabricated using the lithium mixed transition metaloxide of Examples 3 and 5, and LiNiMO₂ of Comparative Examples 2 to 4(M=(Ni_(1/2)Mn_(1/2))_(1−x)Co_(x) and x=0.17 (Comparative Example 5) andx=0.33 (Comparative Example 6), respectively, as a cathode, and alithium metal as an anode. Electrochemical properties of thethus-fabricated coin cells were tested. Cycling was carried outprimarily at 25° C. and 60° C., a charge rate of C/5 and a dischargerate of C/5 (1 C=150 mA/g) from 3 to 4.3 V.

Experimental results of the electrochemical properties for the coincells of Comparative Examples 2 to 4 are given in Table 4 below.Referring to Table 4, the cycle stability was poor with the exception ofComparative Example 3 (Sample B). It is believed that ComparativeExample 4 (Sample C) exhibits the poor cycle stability due to thelithium-deficiency of the surface. Whereas, even though ComparativeExample 2 (Sample A) and Comparative Example 3 (Sample B) were notlithium-deficient, only Comparative Example 3 (Sample B) exhibited a lowcontent of Li₂CO₃. The presence of such Li₂CO₃ may lead to gas evolutionand gradual degradation of the performance (at 4.3 V, Li₂CO₃ slowlydecomposes with the collapse of crystals). That is, there are nonickel-based active materials meeting both the excellent cycle stabilityand the low-impurity content, and therefore it can be shown that nocommercial product is available in which the nickel-based activematerial has excellent cycle stability and high stability againstexposure to air, in conjunction with a low level of Li₂CO₃ impuritiesand low production costs. TABLE 4 Example Sample A Sample B Sample CLiNi_(0.8)Co_(0.2)O₂ Al/Ba-modified AlPO₄-coated Substrate Comp. Ex. 2Comp. Ex. 3 Comp. Ex. 4 Stoichiometric Stoichiometric StoichiometricLi-deficient Li:M at surfaces Li₂CO₃ High High Low impurities Capacityat 193, 175 mAh/g 195, 175 mAh/g 185, 155 25° C. mAh/g C/10, C/1Capacity loss 30% per 100 11% per 100 >30% per cycles cycles 100 cycles

On the other hand, the cells of Comparative Examples 5 and 6 exhibited acrystallographic density of 4.7 and 4.76 g/cm³, respectively, which werealmost the same, and showed a discharge capacity of 157 to 159 mAh/g ata C/10 rate (3 to 4.3 V). Upon comparing with LiCoO₂ having acrystallographic density of 5.04 g/cm³ and a discharge capacity of 157mAh/g, a volume capacity of the cell of Comparative Example 5 is equalto a 93% level of LiCoO₂, and the cell of Comparative Example 6 exhibitsa crystallographic density corresponding to a 94% level of LiCoO₂.Therefore, it can be seen that a low content of Ni results in a poorvolume capacity.

Table 5 below summarizes electrochemical results of coin cells usingLiNiMO₂ in accordance with Example 3 as a cathode, and FIG. 14 depictsvoltage profiles, discharge curves and cycle stability. Acrystallographic density of LiNiMO₂ in accordance with Example 3 was4.74 g/cm³ (cf. LiCoO₂: 5.05 g/cm³). A discharge capacity was more than170 mAh/g (cf. LiCoO₂: 157 mAh/g) at C/20, thus representing that thevolume capacity of LiNiMO₂ was much improved as compared to LiCoO₂.Electrochemical properties of LiNiMO₂ in accordance with Example 5 weresimilar to those of Example 3. TABLE 5 Capacity retention after 100Primary charge cycles (extrapolated) capacity C/5-C/5 cycle, 3.0-4.3 V3.0-4.3 V, C/10 Discharge capacity 25° C. 60° C. — 25° C., 25° C., 60°C., C/1 C/20 C/20 >96% >90% >190 mAh/g 152 mA/g 173 mAh/g 185 mAh/g

Experimental Example 2 Determination of Thermal Stability

In order to examine the thermal stability for the lithium mixedtransition metal oxide of Example 3 and LiNiMO₂ in accordance withComparative Examples 3 and 4, DSC analysis was carried out. Thethus-obtained results are given in FIGS. 15 and 16. For this purpose,coin cells (anode: lithium metal) were charged to 4.3 V, disassembled,and inserted into hermetically sealed DSC cans, followed by injection ofan electrolyte. A total weight of the cathode was from about 50 to about60 mg, A total weight of the electrolyte was approximately the same.Therefore, an exothermic reaction is strongly cathode-limited. The DSCmeasurement was carried out at a heating rate of 0.5 K/min.

As a result, Comparative Example 3 (Al/Ba-modified LiNiO₂) andComparative Example 4 (AlPO₄-coated LiNiO₂) showed the initiation of astrong exothermic reaction at a relatively low temperature.Particularly, Comparative Example 3 exhibited a heat evolution thatexceeds the limit of the device. The total accumulation of heatgeneration was large, i.e. well above 2,000 kJ/g, thus indicating a lowthermal stability (see FIG. 15).

Meanwhile, LiNiMO₂ of Example 3 in accordance with the present inventionexhibited a low total heat evolution, and the initiation of anexothermic reaction at a relatively high temperature as compared toComparative Examples 3 and 4 (see FIG. 16). Therefore, it can be seenthat the thermal stability of LiNiMO₂ in accordance with the presentinvention is excellent.

Experimental Example 3 Test of Electrochemical Properties of PolymerCells with Application of Lithium Mixed Transition Metal Oxide

Using the lithium mixed transition metal oxide of Example 3 as a cathodeactive material, a pilot plant polymer cell of 383562 type wasfabricated. For this purpose, the cathode was mixed with 17% by weightLiCoO₂, and the cathode slurry was an NMP/PVDF-based slurry. Noadditives for the purpose of preventing gelation were added. The anodewas mesocarbon microbead (MCMB) anode. The electrolyte was a standardcommercial electrolyte free of additives known to reduce excessiveswelling. Experiments were carried out at 60° C. and charge anddischarge rates of C/5. A charge voltage was from 3.0 to 4.3 V.

FIG. 17 shows the cycle stability of the battery of the presentinvention (0.8 C charge, 1C discharge, 3 to 4 V, 2 V) at 25° C. Anexceptional cycle stability (91% at C/1 rate after 300 cycles) wasachieved at room temperature. The impedance build up was low. Also, thegas evolution during storage was measured. The results thus obtained aregiven in FIG. 18. During a 4 h-90° C. fully charged (4.2 V) storage, avery small amount of gas was evolved and only a small increase ofthickness was observed. The increase of thickness was within or lessthan the value expected for good LiCoO₂ cathodes tested in similar cellsunder similar conditions. Therefore, it can be seen that LiNiMO₂prepared by the method in accordance with the present invention exhibitsvery high stability and chemical resistance.

Example 6

A mixed hydroxide of Formula MOOH(M=Ni_(4/15)(Mn_(1/2)Ni_(1/2))_(8/15)Cu_(0.2)) as a mixed transitionmetal precursor and Li₂CO₃ were mixed in a molar ratio of Li:M=1.01:1,and the mixture was sintered in air at 900° C. for 10 hours, therebypreparing 50 g of a lithium mixed transition metal oxide having acomposition of LiNi_(0.53)Co_(0.2)Mn_(0.27)O₂.

X-ray analysis was carried out to obtain detailed lattice parameterswith high resolution. Cation mixing was observed by Rietveld refinement.The results thus obtained are given in Table 6 below.

Comparative Example 7

A lithium mixed transition metal oxide was prepared in the same manneras in Example 6, except that a ratio of Li:M was set to 1:1 andsintering was carried out under an O₂ atmosphere. Then, X-ray analysiswas carried out and the cation mixing was observed. The results thusobtained are given in Table 6 below. TABLE 6 Ex. 4 Comp. Ex. 7 Li:M1.01:1 1:1 Unit cell vol. 33.921 Å³ 33.798 Å³ Normalized c:a ratio1.0122 1.0124 c:a/24{circumflex over ( )}0.5 Cation mixing 4.6% 1.5%

As can be seen from Table 6, the lithium mixed transition metal oxide ofExample 6 in accordance with the present invention exhibited a largerunit cell volume and a smaller c:a ratio, as compared to that ofComparative Example 7. Therefore, it can be seen that the lithium mixedtransition metal oxide of Comparative Example 7 exhibited an excessivelylow cation mixing ratio due to the heat treatment under the oxygenatmosphere. This case suffers from deterioration of the structuralstability. That is, it can be seen that the heat treatment under theoxygen atmosphere resulted in the development of a layered structure dueto excessively low cation mixing, but migration of Ni²⁺ ions washindered to an extent that the cycle stability of the battery isarrested.

Example 7

A lithium mixed transition metal oxide having a composition ofLiNi_(0.4)CO_(0.3)Mn_(0.3)O₂ was prepared in the same manner as inExample 6, except that a mixed hydroxide of Formula MOOH(M=Ni_(1/10)(Mn_(1/2)Ni_(1/2))_(6/10)Co_(0.3)) was used as a mixedtransition metal precursor, and the mixed hydroxide and Li₂CO₃ weremixed in a ratio of Li:M=1:1. The cation mixing was observed by X-rayanalysis and Rietveld refinement. The results thus obtained are given inTable 7 below. TABLE 7 Li:M 1:1 Unit cell vol. 33.895 Å³ Normalized c:aratio c:a/24{circumflex over ( )}0.5  1.0123 Cation mixing 3% Capacity(mAh/g) 155   

Example 8

A lithium mixed transition metal oxide having a composition ofLiNi_(0.65)CO_(0.2)Mn_(0.15)O₂ was prepared in the same manner as inExample 6, except that a mixed hydroxide of Formula MOOH(M=Ni_(5/10)(Mn_(1/2)Ni_(1/2))_(3/10)Co_(0.2)) was used as a mixedtransition metal precursor, and the mixed hydroxide and Li₂CO₃ weremixed in a molar ratio of Li:M=1:1. The cation mixing was observed byX-ray analysis and Rietveld refinement. The results thus obtained aregiven in Table 8 below. TABLE 8 Li:M 1:1 Unit cell vol. 34.025 Å³Normalized c:a ratio c:a/24{circumflex over ( )}0.5  1.0107 Cationmixing 7% Capacity (mAh/g) 172   

From the results shown in Tables 7 and 8, it can be seen that thelithium mixed transition metal oxide in accordance with the presentinvention provides desired effects, as discussed hereinbefore, in agiven range.

INDUSTRIAL APPLICABILITY

As apparent from the above description, a method for preparing a lithiummixed transition metal oxide in accordance with the present inventionenables the production of a lithium mixed transition metal oxide havinga given composition and a specific atomic-level structure, by asolid-state reaction of Li₂CO₃ with a mixed transition metal precursorunder an oxygen-deficient atmosphere. Therefore, it is possible toachieve an environmentally friendly preparation method, decreasedproduction costs and improved production efficiency. Since thethus-prepared lithium mixed transition metal oxide exhibits a stablecrystal structure and is substantially free of water-soluble bases suchas lithium carbonates, a secondary battery comprising such a lithiummixed transition metal oxide has a high capacity, excellent cyclecharacteristics, and significantly improved storage properties andhigh-temperature safety.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A method for preparing a lithium mixed transition metal oxide,comprising subjecting Li₂CO₃ and a mixed transition metal precursor to asolid-state reaction under an oxygen-deficient atmosphere with an oxygenconcentration of 10 to 50% by volume to thereby prepare a powderedlithium mixed transition metal oxide having a composition represented byFormula I below:Li_(x)M_(y)O₂  (I) wherein: M=M′_(1−k)A_(k), wherein M′ isNi_(1−a−b)(Ni_(1/2)Mn_(1/2))_(a)Co_(b), 0.65≦a+b≦0.85 and 0.1≦b≦0.4; Ais a dopant; 0≦k≦0.05; and x+y≈2 and 0.95≦x≦1.05.
 2. The methodaccording to claim 1, wherein the oxygen concentration is 10% to 30% byvolume.
 3. The method according to claim 2, wherein the atmosphere is anair atmosphere.
 4. The method according to claim 1, wherein the mixedtransition metal precursor is at least one selected from the groupconsisting of M(OH)₂ and MOOH wherein M is as defined in Formula I. 5.The method according to claim 4, wherein the mixed transition metalprecursor is MOOH, and is prepared by an ammonia-free process.
 6. Themethod according to claim 1, wherein the mixed transition metalprecursor has a tap density of 1.1 to 1.6 g/cm³.
 7. The method accordingto claim 1, wherein a mixing ratio of Li₂CO₃ and the mixed transitionmetal precursor is 0.95 to 1.04:1 wherein the ratio of Li₂CO₃:mixedtransition metal precursor is a w/w ratio.
 8. The method according toclaim 1, wherein the solid-state reaction includes a sintering processat 600 to 1,100° C. for 3 to 20 hours.
 9. The method according to claim8, wherein an amount of air exceeding 2 m³/kg LiMO₂ during the sinteringprocess is supplied to a reaction vessel equipped with a heat exchangerfor pre-warming of the air.
 10. The method according to claim 1, whereinthe lithium mixed transition metal oxide is prepared by a large-scaleprocess of 5 kg or more under a high rate of air circulation of at least2 m³ of air by volume at room temperature per 1 kg of the final lithiummixed transition metal oxide.
 11. The method according to claim 10,wherein for the high rate of air circulation, the air is pumped into orout of the reaction vessel.
 12. The method according to claim 11,wherein at least 10 m³ of air per 1 kg of the final lithium mixedtransition metal oxide is pumped into or out of the reaction vessel. 13.The method according to claim 9, wherein the heat exchanger pre-warmsin-flowing air before the in-flowing air enters the reaction vessel,while cooling the out-flowing air.
 14. A lithium mixed transition metaloxide prepared by the method of claim
 1. 15. A lithium secondary batterycomprising the lithium mixed transition metal oxide of claim 14 as acathode active material.